Although dental caries destroys dental hard tissues, both demineralization as a destructive process and remineralization as a restorative process take place in carious lesions. These conflicting processes occur constantly either simultaneously or alternately. In crystallographic terms, demineralization is crystal dissolution, and remineralization is restoration of partially dissolved crystals, new crystal formation and growth of surviving crystals. In this review, we employ high-resolution transmission electron microscopy to demonstrate demineralization and remineralization of enamel crystals obtained from cross-cut sections of the c-axes of crystals from carious lesions.